Submerged flame evaporator



1959 F. L. HOLTDERREED EIAL 2,867,972

SUBMERGED FLAME EVAPORATOR Filed Dec. 5, 1951 F'IG.I

M 3%. am y E ATTORNEYS r 2,867,972 SUBMERGED FLAME EVAPORATOR Francis Louis Holderreed, Anaconda, and Robert E. Sullivan, Butte, Mont., assignors to The Anaconda Company, a corporation of Montana Application December 5, 1951, Serial No. 259,981 2 Claims. Cl. 159-16) 1 This invention relates to the evaporation of solutions by submerged combustion, aprocedure which involves introducing hot combustion gases beneath the surface of a solution to be evaporated. The invention provides an improved slotted nozzle pipe to obtain thorough distribution of the hot combustion gases into and through evaporator installations. High maintenance costs due to failure of joints as a result of vibration, and the difficulties of prov ding satisfactory vibration-proof mountings for permanent submerged combustion units of commercial size, have been important factors in preventing more widespread adoption of evaporators of this type in preference to more conventional and more complex types of evaporation equipment.

Another feature of submergedv combustion evaporators as heretofore proposed, which'has substantially limited the efficiency of such apparatus, has. been the geysering and excessive agitation that accompanies introduction of combustion gases directly into the body of solution to be evaporated. As a result, hot gases escape from the solution before they have transferred the maximum practicable portion of their heat content to the solution and before they have become substantially saturated with solvent vapor driven 01f from the solution.

The improved submerged combustion nozzle of this invention not only reduces vibration of the apparatus during operation, but also improves its efiiciency by inducing a smoother flow of gases into the solution with .consequent minimization of geysering and turbulence. The new apparatus; comprises a closed evaporation vessel adapted to contain the solution to be evaporated, and a combustion chamber mounted preferably, although not essentially, in a vertical position above such vessel. A cylindrical nozzle pipe, open at both ends, projects substantially vertically down into the evaporation vessel, the upper end of such nozzle pipe opening to the interior of the combustion chamber and the lower end thereof openingto the interior of the evaporation vessel at a point below the normal level of solution therein. The lower end of the nozzle pipe is formed at substantially equal intervals about its circumference with slots, preferably of substantially greater length than width, and preferably but not necessarily of rectangular configuration. These slots may also advantageously be triangular in shape with their apices at the top and their bases at the lower extremity of the nozzle pipe. permit escape of small bubbles of combustion gases at highest pressure through the smallest openings at the top of the slots, the size of the escaping bubbles increasing as 2,867,972 Patented Jan. 13, 1959 solution to be evaporated. The slots are preferably num-v erous enough and long enough so that substantially the entire volume of hot gases delivered through the nozzle pipe passes into the solution through the slots rather than around the lower extremity of the nozzle pipe. By

the provision of such slots, vibrationof the apparatus and geysering of the solution are bothjsubstantially reduced.

An advantageous embodiment of the. invention is described below with reference to the accompanying drawing, in which Fig. l is a vertical section through a submerged com-3 bustion evaporator constructed in accordance with the invention;

. Fig. 2 is a perspective on an enlarged scale of the slotted nozzle pipe employed in the apparatus of Fig.

1; and

Fig. 3 is a perspective on an enlarged scale of an advantageous form of nozzle pipe having triangular slots.

The apparatus shown inFig. 1 comprises a cylindrical combustion chamber 5 positioned vertically abovea closed evaporation vessel 6 having a conical lower por- Triangular slots tion 7 and a cylindrical upper portion 8. The interior of the combustion chamber 5 communicates with the interior of the evaporation vessel 6 through cylindrical nozzle pipe 9 which is secured by bolts 10 passing through a nozzle flange 11 at the upper end of the nozzle into the base structure of the combustion chamber, 5.

The combustion chamber 5 comprises a cylindrical steel shell 12 which is open at its lower end and closed at its upper end by cap; 13. The shell and cap of the chamber are completely lined with a firebrick lining 14, the diameter and cross-section of the chamber inside the lining being substantially the same as the nozzle pipe 9.

Thus the nozzle and the combustion chamber together define a continuous interior passage of substantially uniform diameter and cross section throughout its length, said passage opening at the lower end of the nozzle 9 to the interior of the evaporator vessel 6.

A mixture of gaseous fuel and air is supplied to the interior of the combustion chamber 5, wherein it is burned, through a fuel inlet pipe 15. The fuel-air-mixture is prepared in a mixing chamber 16 to which an air supply pipe 17 and a fuel supply pipe 18 are connected and from which the fuel inlet pipe 15 extends. Ports 19 are provided for inspection of the interior of the combustion chamber, and for the insertion of electrical ignition points to ignite the fuel-air mixture whenever such isrequired.

The evaporation vessel 6 is supported by columns 20.

The nozzle pipe 9 extends about two-thirds, more or less, of the way down into the interior of the vessel 6,

and its lower end portion 25 is formed with a group of slots 26. Fig. 2 shows the slotted lower portion of the nozzle pipe on an enlarged scale. The slots 26 are formed at substantially equally spaced intervals around the circumference of the nozzle, and extend up from the lower They are of about the same width' as the metal fingers 27 which are left between them, and are'dispos'ed essentially parallel'to'the axis of the nozzle'pipe. The'nozzle pipe extends far enough into the evaporation vessel 6 so that the slots at its' lower end' portion are located wholly below the normal solution level 28 defined by the position of the overflow pipe 23.

Fig. 3 shows-the slotted lower portion of an alternative form of: the nozzle pipe 25 on'a'n enlarged'sc'ale. The slots 28 are of triangular configuration and are formed at substantially equally spaced intervals around the-circumference of the-nozzle. Their apices are'spaced asubstantial distance up'from their bases at the lower endo'f'the nozzle pipe. The tops of the metal fingers 29' in the nozzle o'f'Fig. 3 may with advantage be considerablywiderand hence sturdier than in the case of the rectangular metal fingersin the'nozzle of Fig.2.

The use of a slotted nozzle pipe of the character just described has the effect of making the flow of hot combustion gases from the combustion chamber into the solution in the evaporation vessel much smoother than when such gases are simply discharged from the open lower end of a conventional uns'lotted nozzle pipe. There'- by geysering of the solution is substantially eliminated, and vibration of the apparatus very much reduced. Also, distribution of the hot gases through the solution is much improved, with a resulting improvement in the thermal efficiency of the apparatus;

In the operation of an actual evaporator installation using a nozzle having rectangular slots as shown in Fig. Zahdconst'ructed substantially as described above for the'concentration of phosphoric acid, wherein thein terior ofthe combustion chamber and nozzle pipe9 were each some nine inches in diameter, it was found to be possible to evaporated tons of water'per twentyfour'hours, though a rate of ten tons of water evaporated per twenty-four hours appeared best for this installation [the higher rate involved increased stack losses and v undesirably high temperatures in the combustion chamberl. The installation was highly 'efiicient; natural'gas fuel was used and onlyabout 1.5 cubic feet of fuel gas was required per pound of water evaporated.

The introduction of a considerable excess of air with the fuel gas was found to havemarked advantages; it reduced the amount of vibration that occurred in the operation of the apparatus, it resulted in operation at lower temperatures, and it provideda large volume of gas toact as a carrier for theevapo'rated water. Optimum operation and efiicien'cy were obtained using 40 to 50% excess ofairover the amount required for'cornplete combustion of the fuel. It was found best to provide a separate'blower' for the air supply and to supply air to the mixingchamber at a' gauge pressure of 5 pounds per square inch.

Dilute phosphoric acid filtrate, which averaged P 0 content, was fed continuously to the evaporation tank at a rate which with a constant setting of air and fuelfflow to the burner resulted in concentrating the acid infthe evaporation vessel" to a boiling temperature of about 225 F. Itwas found'that when the temperature of the acid in the evaporation vessel was maintained constant, the product was uniform in Baum value and grade. The typical 20% P 0 filtrate was evaporated at 225 F., at 24.5 inch barometer, to 52% P 0 for shipping acid. Evaporation at 210 F., at 24.5 inch barometer, gave 45% P O for use as mixer acid in making treble superphosphate. When the installation was-operatedata rate of,l0 tons' of water evaporated per twenty four hours,

the P 0 loss was about 0.5%. Impingement devices,

scrubbers, or a Cottrell mist treater could, of course, be employed to eliminate atmospheric pollution and recover stack losses. 1

Some 50% of the fluorine in the dilute phosphoric acid solutions evaporated was eliminated, as compared with 30% elimination when concentrating the same solution in conventional Swenson evap'orators. It was found that the quality of the concentrated product was uniformly satisfactory, particularly in that a very close control of Baum value is possible, which is not always the case with the concentrated product from Swenson evaporators.

We claim:

1. In apparatus for the evaporation of solutions by submerged combustion comprising a closed evaporation vessel, a combustion chamber positioned above said vessel, and a cylindrical nozzle pipe open at bothends communicating at' one end with said combustion chamber and at the other end extending 'substantiallyvertically downward into the evaporation vessel,'the improveme'nt which comprises forming the lower end portion of saidnoz'zle in"'the evaporation vessel with a group of "closely spaced slots distributed substantially uniformly about the circumference of the nozzle pipe, saidslots bein'g of triangular configuration with their bases at the lower endof the nozzle pipe and with their apices at a distancesubstantiallygreater'than the width of said base up from the-lower end thereof, the slotted portion of said nozzle pipe being positioned wholly below the normal bu'stion'and air into the solution in streams of bubbles of different size, with the bubbles of smallest size being introduced farthest from the'bottom of the body of solution a'nd with the size of the bubbles progressively increasing toward the bottom of the body of solution.

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